Semiconductor image sensors are used for sensing light. Complementary metal-oxide-semiconductor (CMOS) image sensors (CIS) and charge-coupled device (CCD) sensors are widely used in various applications such as digital still camera or mobile phone camera applications. These devices utilize an array of pixels in a substrate, including photodiodes and transistors, that can absorb radiation projected toward the substrate and convert the sensed radiation into electrical signals.
A back side illuminated (BSI) image sensor device is one type of image sensor device. Fabrication of the BSI image sensor device typically requires a thinning process to reduce the thickness of the substrate. A polishing process is also typically performed after (or as a part of) the thinning process to make sure that the back side of the BSI image sensor device is smooth and flat. However, the thinning process and the polishing process may lead to defects in the BSI image sensor device, especially near the back side of the substrate. These defects may cause dark currents and white pixels, which degrade image quality and performance of the BSI image sensor device. Existing fabrication methods of BSI image sensor devices may not be able to sufficiently address these problems.
Therefore, while existing methods of fabricating BSI image sensor devices have been generally adequate for their intended purposes, they have not been entirely satisfactory in every aspect.